The Cardinals' Carlos Beltran hits a two-run home run during the fourth inning of Game 1 of the National League Championship Series. (Mark Humphrey/AP)

When Trevor Rosenthal removed his St. Louis Cardinals road gray jersey Sunday night, he revealed a blue T-shirt that read, in big block letters, “Cardinals Strength.” It is a slogan, but it also might be a reasonable question, because in Game 1 of the National League Championship Series, they revealed so many aspects that built to a 6-4 victory over the San Francisco Giants.

The Cardinals have seasoned postseason hitters, and two of them, David Freese and Carlos Beltran, unloaded two-run homers. The bottom of the order, middle infielders Daniel Descalso and Pete Kozma, were in the center of the four-run fourth inning that broke it open, because each doubled. Their defense was flawless.

But go back, even if you don’t want to, to Friday’s victory over the Washington Nationals in the decisive fifth game of their National League Division Series. That night, the Cardinals memorably fell behind 6-0, only to come back to win. The reasons for that comeback — Descalso’s game-tying, two-run single and Kozma’s game-winning two-run single — are already stitched in Washington baseball lore, a sordid place to be. But there would have been no comeback without the St. Louis bullpen, perhaps the true “Cardinals Strength.”

“The last two days,” Descalso said, “they’ve been huge for us.”

Sunday night, when starter Lance Lynn was handed a 6-0 lead and grew in danger of blowing it, the relievers came on. Six of them were necessary. “That wasn’t how we wanted to do this,” St. Louis Manager Mike Matheny said. But their contributions: 51 / 3 innings of two-hit, scoreless ball. A game that featured 10 runs before the fourth inning stayed that way.

“Usually,” Cardinals first baseman Allen Craig said, “games like that don’t.”

The NLCS — underway just 44 hours after the devastating events Friday night at Nationals Park — features the last two World Series champions. Each enjoyed an unlikely ride to those championships. But they also are unlikely participants in this NLCS.

The Giants are here not only because they won the National League West, but because they somehow lost the first two games of their division series at home, then went to Cincinnati and beat the Reds in three straight games.

But the Giants were only hosting Game 1 because of the even less likely events Friday in Washington. After wrapping up their series with the Reds last Thursday, the club remained in Cincinnati on Friday, sitting on the tarmac, waiting to find out who they would face — and where. When the Cardinals completed their comeback, the Giants’ charter had to wait some more: It was fueled only for a flight to Washington, and needed gas to get to San Francisco

When they arrived, they had a pitcher any team in the postseason would have liked on the mound, left-hander Madison Bumgarner, a 16-game winner who posted a 2.33 ERA at AT&T Park during the regular season. He would face Lynn, last seen on a baseball diamond allowing Jayson Werth’s game-ending homer in the bottom of the ninth after a 13-pitch at-bat in Game 4 at Nationals Park.

But Bumgarner responded with his shortest outing of the season, and Lynn followed with his second-shortest as a starter, allowing the Giants back into it with four straight scalded balls in the fourth, the last two a triple from Gregor Blanco and a double from Brandon Crawford.

Matheny turned first to Joe Kelly for three outs, then to his only left-hander, Marc Rzepczynksi, for one. Then came the power. Rosenthal, a right-hander who was arguably the best pitcher in the series against Washington, throws 100 mph, and he took over to start the sixth.

“Any time somebody’s throwing 99, 100 miles per hour, that’s not going to be an easy at-bat,” Descalso said.

He issued a two-out walk, but struck out Angel Pagan to close the inning. That brought on Edward Mujica, acquired in a deadline deal with Florida that went somewhat unnoticed.

“It should make a lot of noise,” Descalso said. “That was the only acquisition we made at the deadline. He’s come in and he pitched almost every day for us since he’s been over here, and he’s been outstanding.”

Sunday, he faced the Giants’ Nos. 2, 3 and 4 hitters — Marco Scutaro, Pablo Sandoval and Buster Posey. He struck them all out swinging.

“Right now, we’re just being like a pretty strong bullpen,” said Mujica, who was credited with the victory. “They give the ball to us, and they have confidence. I have big confidence right now in the seventh inning.”

That set up the eighth and ninth exactly as the Cardinals want it. Mitchell Boggs pitched the eighth flawlessly. He turned it over to closer Jason Motte, who pitched the final two innings of Game 5 in Washington. Motte allowed a two-out single to Pagan, but retired Scutaro to end it. In all, the Cardinals’ relievers faced 20 batters and retired 16, with no extra-base hits.

“All you want to do is help out,” Rosenthal said.

They are, for now, more than helping out. They are dominating, and they are a significant reason not only why the Cardinals lead this series, but why they’re here in the first place.